Synthesis and characterization of a new ethynyl-bridged C60 derivative bearing a diketopyrrolopyrrole moiety

Article abstract of DOI:10.1016/j.tetlet.2011.07.029

A new soluble C₆₀ derivative bearing a diketopyrrolopyrrole (DPP) moiety has been synthesized by ethynylation reaction. Characterization by UV-visible spectrophotometry and cyclic voltammetry has been performed in order to study the influence o

Full text of DOI:10.1016/j.tetlet.2011.07.029

Tetrahedron Letters 52 (2011) 5008–5011
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of a new ethynyl-bridged C60 derivative
bearing a diketopyrrolopyrrole moiety
Antoine Lafleur-Lambert a, Simon Rondeau-Gagné a, Armand Soldera b, Jean-François Morin a,
⇑
a Département de chimie, Centre de recherche sur les matériaux avancés, Université Laval, 1045 Ave. de la Médecine, Québec, Canada G1V 0A6
b Département de chimie, Université de Sherbrooke,2500, Boul. de l’Université, Sherbrooke, Québec, Canada J1K 2R1
a r t i c l e i n f o
a b s t r a c t
Article history:
A new soluble C60 derivative bearing a diketopyrrolopyrrole (DPP) moiety has been synthesized by eth-
ynylation reaction. Characterization by UV–visible spectrophotometry and cyclic voltammetry has been
performed in order to study the influence of the DPP moiety on electronic and optical properties of the
fullerene. Moreover, DFT calculations have been conducted on the derivative and its precursor in order
to correlate the orbital geometry with experimental data.
Received 19 April 2011
Revised 5 July 2011
Accepted 8 July 2011
Available online 21 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Fullerene
Diketopyrrolopyrrole
Electrochemistry
DFT calculations
Introduction
found in C60 derivatives used in BHJ solar cells, by an electro-active,
light-harvesting moiety.
Fullerenes, particularly C60, have attracted a lot of attention
since their discovery by Harold, Kroto, and Smalley in 1985.1
Through the combination of relatively low-cost production, ease
of functionalization and astonishing electronic properties, C60 be-
came one of the most promising n-type materials for optoelec-
tronic application.2 However, pristine C60 has poor solubility in
common organic solvents, making large-scale processing difficult
to achieve. This lack of solubility encouraged chemists to develop
Herein we report the synthesis, electrochemical properties and
DFT calculations of a new ethynyl-bridged C60 derivative bearing a
3,6-dithiophen-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione
(DT-DPP) moiety (1, Scheme 1). The DT-DPP unit has not only been
chosen because of its relatively low band-gap and ease of function-
alization, but also because of its high occurrence in recently pre-
pared conjugated polymers for BHJ solar cells.6 It is well accepted
that small organic molecules provide better thin film morphology
when blended with a polymer having structural compatibility.7
To the best of our knowledge, this is the first report of the prepa-
ration of a monosubstituted, unsymmetrical DPP derivative for
opto-electronic applications. It is worth mentioning that Janssen
et al. recently reported the preparation of C60-DPP derivatives.8
However, the C60 units were attached at both ends of a symmetri-
cal DPP unit using a Prato reaction.
new reactions to covalently attach solubilizing groups on C60
,
while keeping its good electronic properties. In this context, the
ethynylation reaction has proven to be very convenient and effi-
cient for many different linkers.3 We have recently established that
the introduction of an ethynyl-bridged substituent on the C60 cage
can modify its LUMO energy level, which makes this functionaliza-
tion strategy very useful for the development of new n-type mate-
rials for bulk heterojunction (BHJ) solar cells.4
In addition to improving charge transport ability, n-type mate-
rials used in BHJ solar cells should also have the capability of
absorbing the sunlight efficiently in order to maximize charge sep-
aration in the active layer. However, C60 absorbs light mainly in the
high-energy region of the sun spectrum, meaning that a low band-
Results and discussion
Synthesis
The synthesis of compound 1 is presented in Scheme 1. DT-DPP
derivative was synthesized using a well-known procedure.9 Start-
ing from DT-DPP, an alkylation reaction using dodecylbromide
and potassium carbonate was performed to afford compound 2
in good yield. It is noteworthy that the purification of compound
2 was rather difficult because the excess of dodecylbromide used
for the alkylation reaction tends to co-elute with compound 2
during the column chromatography. A monobromination reaction
gap
p-conjugated unit has to be installed on C60 in order to
increase sunlight absorption.5 Thus, an interesting strategy is to re-
place the electronically inert solubilizing substituent, commonly
⇑
Corresponding author.
E-mail address: jean-francois.morin@chm.ulaval.ca (J.-F. Morin).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.029

A. Lafleur-Lambert et al. / Tetrahedron Letters 52 (2011) 5008–5011
5009
C12H25
H
N
S
N
S
C12H25Br, K2CO3
1) NBS, CHCl3, 5 min.
O
O
OH
2)
DMF, 120°C, 24h
62%
O
O
S
N
H
S
N
Pd2(dba)3 CHCl3, PPh3,
Et3N, CuI, overnight
66%
C12H25
2
C12H25
C12H25
N
S
N
S
MnO2, KOH, THF
1) C60, LHMDS, THF
O
O
overnight
79%
O
2) TFA
18%
O
OH
S
N
S
N
C12H25
C12H25
3
4
C12H25
O
N
S
H
N
S
O
C12H25
1
Scheme 1. Synthetic pathway to compound 1.
was then performed by adding N-bromosuccinimide (NBS)
(2.1 equiv) to a solution of compound 2 in chloroform, followed
by the addition of distilled water after 5 min of reaction. Although
this short reaction time limits the formation of the bisbrominated
adduct, a significant amount of it was formed and all the purifica-
tion techniques attempted to separate it from the monobromi-
nated adduct failed. Thus, a Sonogashira coupling was carried out
on the mixture using standard conditions with propargyl alcohol
as the alkyne to give compound 3 in 66% yield over two steps.
An alcohol-terminated alkyne was selected in the aim of increasing
the polarity of the coupling products, thus enabling the purification
of the monoadduct by standard column chromatography. Oxida-
tive cleavage of the propargyl alcohol using activated manganese
oxide under anhydrous basic conditions10 was then conducted to
afford the terminal alkyne precursor 4 in good yield.
To the best of our knowledge, no monobromination of a DT-DPP
unit has never been reported in literature. By the time that the pa-
per was under review, a paper describe a different way of prepar-
ing monosubstituted Dt-DPP derivative.11 Thus, another strategy to
brominate the DT-DPP unit was attempted in order to obtain the
monobrominated adduct in pure form in good yield. The strategy
is depicted in Scheme 2. First, alkylation using standard basic con-
ditions with n-octylbromide was conducted. In this case, octyl
chains were chosen to substantially decrease the co-elution phe-
nomenon during the purification process by column chromatogra-
phy. After compound 5 was obtained, monobromination was
conducted using 1.8 equiv of NBS (instead of 2.1 equiv) and the
time of reaction was reduced to 2.5 min. These simple modifica-
tions helped significantly in reducing the amount of bisbrominated
adducts, thus enabling the purification by column chromatogra-
phy. It is noteworthy that the work-up procedure must be carried
out very quickly since reactive NBS remains in the mixture even
Sonogashira coupling using standard conditions was then con-
ducted with trimethylsilylacetylene (TMSA) to afford compound
7 in good yield.
The ethynylation reaction on C60 was conducted using slightly
modified conditions reported by Tour et al.12 Compound 4 and
C60 were added in dried and degassed THF and sonicated for 3 h be-
fore lithium hexamethyldisilylamide (LHMDS) was added drop-
wise to the mixture to give a red–purple solution (characteristic
of the DT-DPP unit and the C60 anion mixture). After few minutes
of stirring at room temperature, trifluoroacetic acid was added to
the mixture and the solvent was immediately removed under re-
duced pressure. A proton was used to quench the C60 anion be-
cause it is more reactive than any of the electrophiles previously
reported to quench this reaction.4,3c Moreover, previously pub-
lished ethynyl-bridged C60 derivatives bearing a proton on the
C60 cage in alpha position relative to the alkyne showed very good
chemical and thermal stability.4 Compound 1 was purified by stan-
dard column chromatography on silica gel. The success of the eth-
ynylation reaction was assessed by 1H NMR. As expected, a peak at
around 7.1 ppm, corresponding to the proton directly attached to
the C60 core, appeared and the peak at 3.59 ppm, corresponding
to the terminal alkyne of compound 4, disappeared. Furthermore,
a set of peaks between 130 and 145 ppm were observed by 13C
NMR, which correspond to the carbon atoms of the C60 core.
In order to study the influence of the DT-DPP moiety on the
LUMO energy level of the C60, cyclic voltammetry measurements
were performed on thin film of compounds 1, 4, and the well-
studied PCBM (C61-butyric methylester) for the sake of comparison.
Results are shown in Figure 1 and summarized in Table 1. For the
thin film formation, a saturated solution of the sample to analyze
was deposited on the working electrode. LUMO levels were calcu-
lated using the following equation: ELUMO = À(Eonset + 4.7 eV).13 It
is noteworthy that our values are not absolute ones, but they were
used to establish comparison between our derivatives.
after chloroform/water liquid extractions. Compound
6 was
obtained as analytically pure material in moderate yield (43%).

5010
A. Lafleur-Lambert et al. / Tetrahedron Letters 52 (2011) 5008–5011
C8H17
H
N
N
S
S
NBS (1.8 eq), CHCl3,
C8H17Br, K2CO3
O
O
2.5 min, rt
43%
O
O
DMF, 120 °C, 24h
55%
S
S
N
H
N
C8H17
5
C8H17
C8H17
N
S
N
S
TMSA, Pd2(dba)3 CHCl3,
O
O
O
O
PPh3, Et3N, CuI, overnight
98%
S
N
S
N
TMS
Br
C8H17
C8H17
6
7
Scheme 2. Improved strategy for the synthesis of a monofunctionalized DPP derivative.
Figure 2. UV–visible spectra of compound 1 and 4.
acceptor (compound 1).14 Interestingly, the inductive effect
between the C60 cage and the DT-DPP unit also affects the HOMO
energy level of DT-DPP. As shown in Table 1, the addition of C60
on the terminal alkyne of compound 4 increased its oxidation
potential by 0.82 V (from 0.96 V for 4 to 1.78 V for 1), indicating
a significant decrease of the electronic density on the DT-DPP unit.
This result is in perfect line with the results obtained in the catho-
dic regime. Together, these results show that it is possible to
modulate, to some extent, the energy levels of C60 using the alkynyl
bridge.
Figure 1. Cyclic voltammograms for compounds 1, 4 and PCBM as recorded on thin
film coated on a platinum wire in 0.1 M TBABF4 in MeCN.
Table 1
Cyclic voltammetry data for compound 1, 4 and PCBM
Compounds
Eox/Ered (V)a
HOMO/LUMO (eV)
PCBM
4
1
n.d/À0.81
0.96/À1.06
1.78/À0.93
n.d/À4.20
To evaluate the possibility of using compound 1 as a n-type light-
harvesting material in BHJ solar cells, the UV–visible absorption
spectra of compound 1 and its free alkyne precursor (compound 4)
were recorded. UV–vis spectrum of compound 1 was recorded in a
diluted solution (2.7 Â 10À6 M) in chloroform and the results are
presented in Figure 2. The spectrum of compound 1 shows a band
centered at 270 nm (27,200 mmolÀ1 cm2) attributed to the C60 cage
and another one at 571 nm (33,100 mmolÀ1 cm2) with a shoulder at
À5.66/À3.95
À6.59/À4.08
a
Potential versus SCE measured by cyclic voltammetry at a scan rate of 100 mV/
s. Measurements performed on a thin film of compound coated on a platinum wire.
Tetra-n-butylammonium tetrafluoroborate 0.1 M in MeCN was used as the
electrolyte.
As shown in Table 1, compound 1 presents a higher LUMO level
than PCBM, meaning that the electronic density onto the C60 cage
in compound 1 is higher than in PCBM. This can be attributed to
the electron-rich nature of the thiophene unit directly attached
to the C60 cage. The increase of the LUMO energy level compared
to the widely used PCBM should be very beneficial for the BHJ solar
cells application since the efficiency of the device is directly related
to the energy difference between the HOMO energy level of the do-
nor (e.g., conjugated polymers) and the LUMO energy level of the
540 nm (29,400 mmolÀ1 cm2) attributed to the
p?
p⁄ transition
originating from the DT-DPP moiety. As observed by others and our-
selves, the attachment of a C60 cage onto compound 4 leads to a
slight bathochromic shift of the pp⁄ transition (9 nm) due to an
extension of the effective conjugation length.4,15 This interesting
property is expected to provide increased photon absorption of the
conjugated polymer/fullerene derivative blends that could poten-
tially lead to better BHJ solar cell efficiencies.5

A. Lafleur-Lambert et al. / Tetrahedron Letters 52 (2011) 5008–5011
5011
Acknowledgments
We thank the National Science and Engineering Council of
Canada (NSERC) through the NRC-BDC-NSERC Initiative program
for funding, and the Centre Québécois sur les Matériaux Fonction-
nels (CQMF). Simon Rondeau-Gagné thanks the NSERC for PhD
scholarship. We thank David Gendron (U. Laval) and Pierre-Olivier
Morin (U. Laval) for help in DT-DPP synthesis and for thin film cyclo-
voltammetry. We also thank Prof. M. Leclerc (U. Laval) for helpful
discussions and Philippe Dufour (U. Laval) for HRMS measurements.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.07.029.
References and notes
Figure 3. Calculated molecular orbitals for compounds 1 and 4.
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Conclusions
In summary, we report the synthesis of a monosubstituted 3,6-
dithiophen-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione deriv-
ative bearing
electrochemical and optical properties and an increased LUMO en-
ergy level compared to the well-known PCBM. This last feature
makes this material very promising for the further development
of efficient BHJ solar cells and testing in such devices using this
a C60. This new material exhibits interesting
material in combination with p-conjugated polymers is underway.
16. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.